Because electric power generally is available at 120 VAC domestically, but at 240 VAC in other countries, particularly European nations, electronic equipment used in those respective geographic locations are expected to have power requirements at those respective voltages. In turn, AC power supplies for power protection, such as the no-called "uninterruptible" power supply ("U.P.S."), generally must have the respective voltage output to service those geographic locations. As contemporary U.P.S. systems convert line AC to DC and then "rebuild" a "clean" AC signal using inverters, the inverters must provide either a 120 VAC or 240 VAC output in the respective geographic location.
Rather than have different designs for these inverters, adding to the cost of development, manufacturing and stocking, it is desirable to have DC/AC inverters that may be configured to either 120 VAC or 240 VAC operations with a minimum of change of components. Achievement of this goal also allows for field reconfiguration of the power supplies.
Two approaches have been explored to achieve this goal. The first approach, shown in FIG. 1, is a traditional full bridge inverter in which the DC voltage is supplied by a battery or DC storage capacitor 10, and a controller 20 signals switches 30 and 60 (typically MOSFETS) to open and close alternately with switches 40 and 50. The load 90 therefore sees a "full wave" AC signal, smoothed by inductor 70 and capacitor 80. Some fundamental equations governing the input/output relationship of this inverter are EQU V.sub.out =V.sub.in (2D-1)
and EQU VA.sub.out =I.sub.orms V.sub.orms
where
V.sub.out =instantaneous output voltage PA1 V.sub.in =DC input voltage PA1 D=duty cycle=time switches on/period PA1 VA.sub.out =output volt amperes PA1 I.sub.orms =RMS output current PA1 V.sub.orms =RMS output voltage
As evident from these relationships, to operate with both a 120 VAC and 240 VAC output voltage and produce the same VA, the inverter most generate twice as much current in the 120 VAC mode as in the 240 VAC mode. Thus, all components must be sized to handle the greater current seen during the 120 VAC operating mode. Second, because D is restricted to a value between 0 and 1, the DC input voltage input voltage must be greater than the peak of the AC output voltage plus some margin.
To operate in both modes, the 240 VAC (340 V.sub.peak) mode defines the minimum DC input voltage. This is significant because, when operating at 120 VAC, the maximum duty cycle D will not approach 1, but will be significantly (20-30%) less. This means that the bridge switches must not only conduct greater currents at 120 VAC operation, but carry the current with less conduction time. This has a significant impact on the conduction losses in the bridge switches.
A second approach is depicted in FIG. 2. This method utilizes two half bridge inverters 200 and 250 with isolated sources 201 and 251, such that DC sources 202 and 252 respectively are connected to outputs 235 and 285 alternately with and in opposite polarity to sources 203 and 253, according to the alternate closing of switches 210 and 260 and of 211 and 261. For 120 VAC operation, outputs 235 and 285 are connected, and outputs 240 and 290 are connected to drive a load in parallel. For 240 VAC operation, outputs 240 and 285 are connected in a series connection, and the load is driven off outputs 235 and 290. The same equations set forth above for the first approach apply to each bridge section.
This second approach would allow each bridge to be optimized relative to input voltage and rms current stresses. However, the methods for controlling the output voltage and guaranteeing the equal share of power between the two bridges present a complex control problem. Moreover, it became apparent that different methods of control were required for each of the 120 VAC and 240 VAC modes of operation, defeating the goal of maximizing the commonality of control circuitry.